Method of Coding and Decoding Images, Coding and Decoding Device and Computer Programs Corresponding Thereto

ABSTRACT

A method for coding includes; segmenting an image into blocks; grouping blocks into a number of subsets; coding, using an entropy coding module, each subset, by associating digital information with symbols of each block of a subset, including, for the first block of the image, initializing state variables of the coding module; and generating a data sub-stream representative of at least one of the coded subsets of blocks. Where a current block is the first block to be coded of a subset, symbol occurrence probabilities for the first current block are determined based on those for a coded and decoded predetermined block of at least one other subset. Where the current block is the last coded block of the subset: writing, in the sub-stream representative of the subset, the entire the digital information associated with the symbols during coding of the blocks of the subset, and implementing the initializing sub-step.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 National Stage Application ofInternational Application No. PCT/FR2012/051391, filed Jun. 20, 2012,which is incorporated by reference in its entirety and published as WO2012/175870 on Dec. 27, 2012, not in English.

FIELD OF THE INVENTION

The present invention pertains generally to the field of imageprocessing, and more precisely to the coding and to the decoding ofdigital images and of sequences of digital images.

The invention can thus, in particular, be applied to video codingimplemented in current video coders (MPEG, H.264, etc.) or forthcomingvideo coders (ITU-T/VCEG (H.265) or ISO/MPEG (HVC)).

BACKGROUND OF THE INVENTION

Current video coders (MPEG, H264, etc.) use a block-wise representationof the video sequence. The images are segmented into macro-blocks, eachmacro-block is itself segmented into blocks and each block, ormacro-block, is coded by intra-image or inter-image prediction. Thus,certain images are coded by spatial prediction (intra prediction), whileother images are coded by temporal prediction (inter prediction) withrespect to one or more coded-decoded reference images, with the aid of amotion compensation known by the person skilled in the art. Moreover,for each block can be coded a residual block corresponding to theoriginal block minus a prediction. The coefficients of this block arequantized, possibly after a transformation, and then coded by an entropycoder.

Intra prediction and inter prediction require that certain blocks whichhave been previously coded and decoded be available, so as to be used,either at the decoder or at the coder, to predict the current block. Aschematic example of a predictive coding such as this is represented inFIG. 1, in which an image I_(N) is divided into blocks, a current blockMB_(i) of this image being subjected to a predictive coding with respectto a predetermined number of three previously coded and decoded blocksMBr₁, MBr₂ and MBr₃, such as designated by the hatched arrows. Theaforementioned three blocks specifically comprise the block MBr₁situated immediately to the left of the current block MB_(i), and thetwo blocks MBr₂ and MBr₃ situated respectively immediately above andabove and to the right of the current block MB_(i).

The entropy coder is of more particular interest here. The entropy coderencodes the information in its order of arrival. Typically a row by rowtraversal of the blocks is carried out, of “raster-scan” type, asillustrated in FIG. 1 by the reference PRS, starting from the block atthe top left of the image. For each block, the various items ofinformation necessary for the representation of the block (type ofblock, mode of prediction, residual coefficients, etc.) are dispatchedsequentially to the entropy coder.

An efficient arithmetic coder of reasonable complexity, called “CABAC”(“Context Adaptive Binary Arithmetic Coder”), introduced into the AVCcompression standard (also known by the name ISO-MPEG4 part 10 and ITU-TH.264) is already known.

This entropy coder implements various concepts:

-   -   arithmetic coding: the coder, such as described initially in the        document J. Rissanen and G. G. Langdon Jr, “Universal modeling        and coding,” IEEE Trans. Inform. Theory, vol. IT-27, pp. 12-23,        January 1981, uses, to code a symbol, a probability of        occurrence of this symbol;    -   adaptation to context: here this entails adapting the        probability of occurrence of the symbols to be coded. On the one        hand, learning is carried out on the fly. On the other hand,        depending on the state of the previously coded information, a        specific context is used for the coding. To each context there        corresponds an inherent probability of occurrence of the symbol.        For example a context corresponds to a type of symbol coded (the        representation of a coefficient of a residual, signaling of        coding mode, etc.) according to a given configuration, or a        state of the neighborhood (for example the number of “intra”        modes selected in the neighborhood, etc.);    -   binarization: a shaping of a series of bits of the symbols to be        coded is carried out. Subsequently, these various bits are        dispatched successively to the binary entropy coder.

Thus, this entropy coder implements, for each context used, a system forlearning the probabilities on the fly with respect to the symbols codedpreviously for the context under consideration. This learning is basedon the order of coding of these symbols. Typically, the image istraversed according to an order of “raster-scan” type, describedhereinabove.

During the coding of a given symbol b that may equal 0 or 1, thelearning of the probability p_(i) of occurrence of this symbol isupdated for a current block MB_(i) in the following manner

${p_{i}\left( {b = 0} \right)} = {{\alpha \cdot {p_{i - 1}\left( {b = 0} \right)}} + \left\{ \begin{matrix}\left( {1 - \alpha} \right) & {{if}\mspace{14mu} {coded}\mspace{14mu} {bit}\mspace{14mu} {is}\mspace{14mu} 0} \\0 & {otherwise}\end{matrix} \right.}$

where a is a predetermined value, for example 0.95 and p_(i-1) is thesymbol occurrence probability calculated upon the last occurrence ofthis symbol.

A schematic example of such an entropy coding is represented in FIG. 1,in which a current block MB_(i) of the image I_(N) is subjected to anentropy coding. When the entropy coding of the block MB_(i) begins, thesymbol occurrence probabilities used are those obtained after coding ofa previously coded and decoded block, which is the one which immediatelyprecedes the current block MB_(i) in accordance with the aforementionedrow by row traversal of the blocks of “raster scan” type. Such alearning based on block to block dependency is represented in FIG. 1 forcertain blocks only for the sake of clarity of the figure, by theslender arrows.

A drawback of such a type of entropy coding resides in the fact that,when coding a symbol situated at the start of a row, the probabilitiesused correspond mainly to those observed for the symbols situated at theend of the previous row, having regard to the “raster scan” traversal ofthe blocks. Now, on account of the possible spatial variation of thesymbol probabilities (for example for a symbol related to an item ofmotion information, the motion situated on the right part of an imagemay be different from that observed on the left part and thereforelikewise for the ensuing local probabilities), a lack of localconformity of the probabilities may be observed, thereby possibly givingrise to a loss of efficiency during coding.

To limit this phenomenon, proposals for modifications of the order oftraversal of the blocks have been made, with the aim of ensuring betterlocal consistency, but the coding and the decoding remain sequential.

Therein lies another drawback of this type of entropy coder. Indeed, thecoding and the decoding of a symbol being dependent on the state of theprobability learned thereto, the decoding of the symbols can only bedone in the same order as that used during coding. Typically, thedecoding can then only be sequential, thus preventing parallel decodingof several symbols (for example to profit from multi-corearchitectures).

The document: Thomas Wiegand, Gary J. Sullivan, Gisle Bjontegaard, andAjay Luthra, “Overview of the H.264/AVC Video Coding Standard”, IEEETransactions on Circuits and Systems for Video Technology, Vol. 13, No.7, pp. 560-576, July 2003, point out moreover that the CABAC entropycoder has the particular feature of assigning a non-integer number ofbits to each symbol of a current alphabet to be coded, this beingadvantageous for symbol occurrence probabilities of greater than 0.5.Specifically, the CABAC coder waits until it has read several symbols,and then assigns to this set of symbols read a predetermined number ofbits that the coder writes to the compressed stream to be transmitted tothe decoder. Such a provision thus makes it possible to “mutualize” thebits on several symbols and to code a symbol on a fractional number ofbits, this number reflecting information which is closer to theinformation actually transported by a symbol. Other bits associated withthe symbols read are not transmitted in the compressed stream but arekept on standby while waiting to be assigned to one or more new symbolsread by the CABAC coder making it possible again to mutualize theseother bits. In a known manner, the entropy coder undertakes, at a giveninstant, an “emptying” of these untransmitted bits. Stated otherwise, atsaid given instant, the coder extracts the bits not yet transmitted andwrites them to the compressed stream destined for the decoder. Suchemptying takes place for example at the instant at which the last symbolto be coded has been read, so as to ensure that the compressed streamdoes indeed contain all the bits which will allow the decoder to decodeall the symbols of the alphabet. In a more general manner, the instantat which the emptying is performed is determined as a function of theperformance and functionalities specific to a given coder/decoder.

The document, which is available at the Internet addresshttp://research.microsoft.comlen-us/um/people/jinVlpaper_2002/msri_jpeg.htmon the date of 15 Apr. 2011, describes a method for coding still imagescompliant with the JPEG2000 compression standard. According to thismethod, the still image data undergo a discrete wavelet transformfollowed by a quantization, thereby making it possible to obtainquantized wavelet coefficients with which are respectively associatedquantization indices. The quantization indices obtained are coded withthe aid of an entropy coder. The quantized coefficients are previouslygrouped into rectangular blocks called code-blocks, typically 64×64 or32×32 in size. Each code-block is thereafter coded independently byentropy coding. Thus, the entropy coder, when it undertakes the codingof a current code-block, does not use the symbol occurrenceprobabilities calculated during the coding of previous code-blocks. Theentropy coder is therefore in an initialized state at each start ofcoding of a code-block. Such a method exhibits the advantage of decodingthe data of a code-block without having to decode the neighboringcode-blocks. Thus for example, a piece of client software may request apiece of server software to provide the compressed code-blocks neededsolely by the client to decode an identified sub-part of an image. Sucha method also presents the advantage of permitting the parallel encodingand/or decoding of the code-blocks. Thus, the smaller the size of thecode-blocks, the higher the level of parallelism. For example, for alevel of parallelism fixed at two, two code-blocks will be coded and/ordecoded in parallel. In theory, the value of the level of parallelism isequal to the number of code-blocks to be coded of the image. However,the compression performance obtained with this method is not optimalhaving regard to the fact that such coding does not exploit theprobabilities arising from the immediate environment of the currentcode-block.

SUMMARY OF THE INVENTION

A subject of the present invention relates to a method for coding atleast one image comprising the steps of:

-   -   segmenting the image into a plurality of blocks able to contain        symbols belonging to a predetermined set of symbols,    -   grouping of blocks into a predetermined number of subsets of        blocks,    -   coding, by means of an entropy coding module, of each of the        subsets of blocks, by associating digital information with the        symbols of each block of a subset under consideration, the        coding step comprising, for the first block of the image, a        sub-step of initializing state variables of the entropy coding        module,    -   generation of at least one data sub-stream representative of at        least one of the coded subsets of blocks.

The method according to the invention is noteworthy in that:

-   -   in the case where the current block is the first block to be        coded of a subset under consideration, there is undertaken the        determination of probabilities of symbol occurrence for the        first current block, the probabilities being those which have        been determined for a coded and decoded predetermined block of        at least one other subset,    -   in the case where the current block is the last coded block of        the subset under consideration there is undertaken:        -   the writing, to the sub-stream representative of the subset            under consideration, of the entirety of the digital            information which has been associated with the symbols            during the coding of the blocks of the subset under            consideration,        -   the implementation of the initialization sub-step.

The writing step mentioned hereinabove amounts to performing, as soon asthe last block of a subset of blocks has been coded, an emptying of thedigital information (bits) not yet transmitted, as was explained abovein the description.

The coupling of the aforementioned writing step and of the step ofreinitializing the entropy coding module makes it possible to produce acoded data stream containing various data sub-streams correspondingrespectively to at least one coded subset of blocks, said stream beingadapted for being decoded in parallel according to various levels ofparallelism, and this independently of the type of coding, sequential orparallel, which has been applied to the subsets of blocks. Thus, a largedegree of freedom can be obtained on decoding regarding the choice ofthe level of parallelism, as a function of the coding/decodingperformance expected. The level of parallelism on decoding is variableand can even be different from the level of parallelism on coding, sincewhen commencing the decoding of a subset of blocks, the decoder isalways in an initialized state.

According to a first example, the state variables of the entropy codingmodule are the two bounds of an interval representative of theprobability of occurrence of a symbol from among the symbols of thepredetermined set of symbols.

According to a second example, the state variables of the entropy codingmodule are the strings of symbols contained in the translation table ofan LZW (Lempel-Ziv-Welch) entropy coder, well known to the personskilled in the art, and described at the following Internet address onthe date of 21 Jun. 2011: httpJ/en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch.

The main advantage of using the symbol occurrence probabilitiesdetermined for the first block of said other subset during the entropycoding of the first current block of a considered subset of blocks is toeconomize on the buffer memory of the coder by storing in the lattersolely the updating of said symbol occurrence probabilities, withouttaking into account the symbol occurrence probabilities learned by theother consecutive blocks of said other subset.

The main advantage of using the symbol occurrence probabilitiesdetermined for a block of said other subset, other than the first block,for example the second block, during the entropy coding of the firstcurrent block of a considered subset of blocks is the obtaining of moreprecise and therefore better learning of the probabilities of occurrenceof symbols, thereby giving rise to better video compression performance.

In a particular embodiment, the subsets of blocks are coded sequentiallyor else in parallel.

The fact that the subsets of blocks are coded sequentially has theadvantage of rendering the coding method according to the inventioncompliant with the H.264/MPEG-4 AVC standard.

The fact that the subsets of blocks are coded in parallel has theadvantage of accelerating the coder processing time and of benefitingfrom a multiplatform architecture for the coding of an image.

In another particular embodiment, when at least two subsets of blocksare coded in parallel with at least one other subset of blocks, the atleast two coded subsets of blocks are contained in the same datasub-stream.

Such a provision makes it possible in particular to economize on thesignaling of the data sub-streams. Indeed, so that a decoding unit candecode a sub-stream as early as possible, it is necessary to indicate inthe compressed file the point at which the sub-stream in questionbegins. When several subsets of blocks are contained in the same datasub-stream, a single indicator is necessary, thereby reducing the sizeof the compressed file.

In yet another particular embodiment, when the coded subsets of blocksare intended to be decoded in parallel in a predetermined order, thedata sub-streams delivered after coding respectively of each of thesubsets of blocks are first ordered according to the predetermined orderbefore being transmitted with a view to their decoding.

Such a provision makes it possible to adapt the coded data stream to aspecific type of decoding without needing to decode and then re-encodethe image.

Correlatively, the invention relates further to a device for coding atleast one image comprising:

-   -   means for segmenting the image into a plurality of blocks able        to contain symbols belonging to a predetermined set of symbols,    -   means for grouping the blocks into a predetermined number of        subsets of blocks,    -   means for coding each of the subsets of blocks, the coding means        comprising an entropy coding module able to associate digital        information with the symbols of each block of a subset under        consideration, the coding means comprising, for the first block        of the image, sub-means for initializing state variables of the        entropy coding module,    -   means for generating at least one data sub-stream representative        of at least one of the coded subsets of blocks.

Such a coding device is noteworthy in that it comprises:

-   -   means for determining probabilities of symbol occurrence for a        current block which, in the case where the current block is the        first block to be coded of a subset under consideration,        determine the probabilities of symbol occurrence for the first        block as being those which have been determined for a coded and        decoded predetermined block of at least one other subset,    -   writing means which, in the case where the current block is the        last coded block of the subset under consideration, are        activated to write, to the sub-stream representative of the        subset under consideration, the entirety of the digital        information which has been associated with the symbols during        the coding of the blocks of the subset under consideration,        the initialization sub-means being furthermore activated to        reinitialize the state variables of the entropy coding module.

In a corresponding manner, the invention also relates to a method fordecoding a stream representative of at least one coded image, comprisingthe steps of:

-   -   identification in the stream of a predetermined number of data        sub-streams corresponding respectively to at least one subset of        blocks to be decoded, the blocks being able to contain symbols        belonging to a predetermined set of symbols,    -   decoding of the identified subsets of blocks by means of an        entropy decoding module, by reading, in at least one of the        identified sub-streams, digital information associated with the        symbols of each block of the subset corresponding to said at        least one identified sub-stream, the decoding step comprising,        for the first block to be decoded of the image, a sub-step of        initializing state variables of the entropy decoding module.

Such a decoding method is noteworthy in that:

-   -   in the case where the current block is the first block to be        decoded of a subset under consideration, there is undertaken the        determination of probabilities of symbol occurrence for the        first block of the subset under consideration, the probabilities        being those which have been determined for a decoded        predetermined block of at least one other subset,    -   in the case where the current block is the last decoded block of        the subset under consideration, there is undertaken the        implementation of the initialization sub-step.

In a particular embodiment, the subsets of blocks are decodedsequentially or else in parallel.

In another particular embodiment, when at least two subsets of blocksare decoded in parallel with at least one other subset of blocks, one ofthe identified data sub-streams is representative of the at least twosubsets of blocks.

In yet another particular embodiment, when the coded subsets of blocksare intended to be decoded in parallel in a predetermined order, thedata sub-streams corresponding respectively to the coded subsets ofblocks are previously ordered in said predetermined order in said streamto be decoded.

Correlatively, the invention further relates to a device for decoding astream representative of at least one coded image, comprising:

-   -   means for identification in the stream of a predetermined number        of data sub-streams corresponding respectively to at least one        subset of blocks to be decoded, the blocks being able to contain        symbols belonging to a predetermined set of symbols,    -   means for decoding the subsets of blocks identified, the        decoding means comprising an entropy decoding module able to        read, in at least one of the identified sub-streams, digital        information associated with the symbols of each block of the        subset corresponding to said at least one identified sub-stream,        the decoding means comprising, for the first block to be decoded        of the image, sub-means for initializing state variables of the        entropy decoding module.

Such a decoding device is noteworthy in that it comprises means fordetermining probabilities of symbol occurrence for a current blockwhich, in the case where the current block is the first block to bedecoded of a subset under consideration, determine the probabilities ofsymbol occurrence for the first block as being those which have beendetermined for a decoded predetermined block of at least one othersubset,

and in that in the case where the current block is the last decodedblock of the subset under consideration, the initialization sub-meansare activated to reinitialize the state variables of the entropydecoding module.

The invention is also aimed at a computer program comprisinginstructions for the execution of the steps of the coding or decodingmethod hereinabove, when the program is executed by a computer.

Such a program can use any programming language, and be in the form ofsource code, object code, or of code intermediate between source codeand object code, such as in a partially compiled form, or in any otherdesirable form.

Yet another subject of the invention is also aimed at a recording mediumreadable by a computer, and comprising computer program instructionssuch as mentioned hereinabove.

The recording medium can be any entity or device capable of storing theprogram. For example, such a medium can comprise a storage means, suchas a ROM, for example a CD ROM or a microelectronic circuit ROM, or elsea magnetic recording means, for example a diskette (floppy disk) or ahard disk.

Moreover, such a recording medium can be a transmissible medium such asan electrical or optical signal, which can be conveyed via an electricalor optical cable, by radio or by other means. The program according tothe invention can in particular be downloaded from a network of Internettype.

Alternatively, such a recording medium can be an integrated circuit inwhich the program is incorporated, the circuit being adapted forexecuting the method in question or to be used in the execution of thelatter.

The coding device, the decoding method, the decoding device and thecomputer programs aforementioned exhibit at least the same advantages asthose conferred by the coding method according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages will become apparent on reading twopreferred embodiments described with reference to the figures in which:

FIG. 1 represents an image coding diagram of the prior art,

FIG. 2A represents the main steps of the coding method according to theinvention,

FIG. 2B represents in detail the coding implemented in the coding methodof FIG. 2A,

FIG. 3A represents a first embodiment of a coding device according tothe invention,

FIG. 3B represents a coding unit of the coding device of FIG. 3A,

FIG. 3C represents a second embodiment of a coding device according tothe invention,

FIG. 4A represents an image coding/decoding diagram according to a firstpreferential embodiment,

FIG. 4B represents an image coding/decoding diagram according to asecond preferential embodiment,

FIG. 5A represents the main steps of the decoding method according tothe invention,

FIG. 5B represents in detail the decoding implemented in the decodingmethod of FIG. 5A,

FIG. 6A represents an embodiment of a decoding device according to theinvention,

FIG. 6B represents a decoding unit of the decoding device of FIG. 6A,

FIG. 7A represents an image coding/decoding diagram implementing acoding of sequential type and a decoding of parallel type,

FIG. 7B represents an image coding/decoding diagram implementing acoding/decoding of parallel type, with respectively different levels ofparallelism.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS A First Embodiment ofthe Coding Part

An embodiment of the invention will now be described, in which thecoding method according to the invention is used to code a sequence ofimages according to a binary stream much like that obtained by a codingaccording to the H.264/MPEG-4 AVC standard. In this embodiment, thecoding method according to the invention is for example implemented in asoftware or hardware manner by modifications of a coder initiallycompliant with the H.264/MPEG-4 AVC standard. The coding methodaccording to the invention is represented in the form of an algorithmcomprising steps C1 to C5, represented in FIG. 2A.

According to the embodiment of the invention, the coding methodaccording to the invention is implemented in a coding device CO, twoembodiments of which are represented respectively in FIGS. 3A and 3C.

With reference to FIG. 2A, the first coding step C1 is the segmenting ofan image IE of a sequence of images to be coded into a plurality ofblocks or macro-blocks MB_(i) as represented in FIG. 4A or 4B. Saidmacro-blocks are able to contain one or more symbols, said symbolsforming part of a predetermined set of symbols. In the examplesrepresented, said blocks MB have a square shape and all have the samesize. As a function of the size of the image which is not necessarily amultiple of the size of the blocks, the last blocks on the left and thelast blocks at the bottom may not be square. In an alternativeembodiment, the blocks can be for example of rectangular size and/or notaligned with one another.

Each block or macroblock can moreover itself be divided into sub-blockswhich are themselves subdividable.

Such a segmenting is performed by a partitioning module PCO representedin FIG. 3A which uses for example a partitioning algorithm well known assuch.

With reference to FIG. 2A, the second coding step C2 is the grouping ofthe aforementioned blocks into a predetermined number P of subsets ofconsecutive blocks SE1, SE2, . . . , SEk, . . . , SEP intended to becoded sequentially or in parallel. In the examples represented in FIGS.4A and 4B, P=6, but only four subsets SE1, SE2, SE3, SE4 are representedfor the sake of clarity of the figures. These four subsets of blocks areeach represented dashed and consist respectively of the first four rowsof blocks of the image IE.

Such a grouping is performed by a calculation module GRCO represented inFIG. 3A, with the aid of an algorithm well known per se.

With reference to FIG. 2A, the third coding step C3 consists in thecoding of each of said subsets of blocks SE1 to SE6, the blocks of asubset under consideration being coded according to a predeterminedorder of traversal PS, which is for example of sequential type. In theexamples represented in FIGS. 4A and 4B, the blocks of a current subsetSEk (1≤k≤P) are coded one after another, from left to right, asindicated by the arrow PS.

According to a first variant, such a coding is of the sequential typeand is implemented by a single coding unit UC such as represented inFIG. 3A. In a manner known per se, the coder CO comprises a buffermemory MT which is adapted to contain the symbols occurrenceprobabilities such as progressively re-updated in tandem with the codingof a current block.

As represented in greater detail in FIG. 3B, the coding unit UCcomprises:

-   -   a module for predictive coding of a current block with respect        to at least one previously coded and decoded block, denoted MCP;    -   a module for entropy coding of said current block by use of at        least one probability of symbol occurrence calculated for said        previously coded and decoded block, denoted MCE.

The predictive coding module MCP is a software module which is able toperform a predictive coding of the current block, according toconventional prediction techniques, such as for example in Intra and/orInter mode.

The entropy coding module MCE is for its part of CABAC type, butmodified according to the present invention, as will be describedfurther on in the description.

As a variant, the entropy coding module MCE could be a Huffman coderknown per se.

In the examples represented in FIGS. 4A and 4B, the unit UC codes theblocks of the first row SE1, from left to right. When it reaches thelast block of the first row SE1, it passes to the first block of thesecond row SE2. When it reaches the last block of the second row SE2, itpasses to the first block of the third row SE3. When it reaches the lastblock of the third row SE3, it passes to the first block of the fourthrow SE4, and so on and so forth until the last block of the image IE iscoded.

Other types of traversal than that which has just been describedhereinabove are of course possible. Thus, it is possible to segment theimage IE into several sub-images and to apply a segmenting of this typeto each sub-image independently. It is also possible for the coding unitto process not a succession of rows, as explained hereinabove, but asuccession of columns. It is also possible to traverse the rows orcolumns in either direction.

According to a second variant, such a coding is of the parallel type andis distinguished from the first variant of sequential coding, solely bythe fact that it is implemented by a predetermined number R of codingunits UCk (1≤k≤R), with R=2 in the example represented in FIG. 3C. Suchparallel coding is known to engender a substantial acceleration of thecoding method.

Each of the coding units UCk is identical to the coding unit UCrepresented in FIG. 3B. In a corresponding manner, a coding unit UCkcomprises a predictive coding module MCPk and an entropy coding moduleMCEk.

Again with reference to FIGS. 4A and 4B, the first unit UC1 codes forexample the blocks of the rows of odd rank, while the second unit UC2codes for example the blocks of the rows of even rank. More precisely,the first unit UC1 codes the blocks of the first row SE1, from left toright. When it reaches the last block of the first row SE1, it passes tothe first block of the (2n+1)^(th) row, that is to say the third rowSE3, etc. In parallel with the processing performed by the first unitUC1, the second unit UC2 codes the blocks of the second row SE2, fromleft to right. When it reaches the last block of the second row SE2, itpasses to the first block of the (2n)^(th) row, here the fourth row SE4,etc. The aforementioned two traversals are repeated until the last blockof the image IE is coded.

With reference to FIG. 2A, the fourth coding step C4 is the productionof L sub-streams F1, F2, . . . , Fm, . . . , FL (1≤m≤L≤P) of bitsrepresenting the processed blocks compressed by the aforementionedcoding unit UC or each of the aforementioned coding units UCk, as wellas a decoded version of the processed blocks of each subset SEk. Thedecoded processed blocks, denoted SED1, SED2, . . . , SEDk, . . . ,SEDP, of a subset under consideration may be reused by the coding unitUC represented in FIG. 3A or each of the coding units UCk represented inFIG. 3C, according to a synchronization mechanism which will be detailedfurther on in the description.

With reference to FIG. 3B, the step of producing L sub-streams isimplemented by a stream generating software module MGSF or MGSFk whichis adapted for producing data streams, such as bits for example.

With reference to FIG. 2A, the fifth coding step C5 consists inconstructing a global stream F on the basis of the aforementioned Lsub-streams F1, F2, . . . , Fm, . . . , FL. According to one embodiment,the sub-streams F1, F2, . . . , Fm, . . . , FL are simply juxtaposed,with an extra item of information intended to indicate to the decoderthe location of each sub-stream Fm in the global stream F. The latter isthereafter transmitted by a communication network (not represented), toa remote terminal. The latter comprises the decoder DO represented inFIG. 5A. According to another embodiment, which is particularlyadvantageous since it does not require a decoding and then a re-encodingof the image, the coder CO, before transmitting the stream F to thedecoder DO, previously orders the L sub-streams F1, F2, . . . , Fm, . .. , FL in a predetermined order which corresponds to the order in whichthe decoder DO is able to decode the sub-streams.

Thus, as will be described in detail further on in the description, thedecoder according to the invention is able to isolate the sub-streamsF1, F2, . . . , Fm, . . . , FL within the global stream F and to assignthem to one or more decoding units of which the decoder is composed. Itwill be noted that such a decomposition of the sub-streams in a globalstream is independent of the choice of the use of a single coding unitor else of several coding units operating in parallel, and that it ispossible with this approach to have solely the coder or solely thedecoder which comprises units operating in parallel.

Such a construction of the global stream F is implemented in a streamconstruction module CF, such as represented in FIG. 3A and FIG. 3C.

The various specific sub-steps of the invention, such as are implementedduring the aforementioned step C3 of coding, in a coding unit UC or UCk,will now be described with reference to FIG. 2B.

In the course of a step C31, the coding unit UC or UCk selects ascurrent block the first block to be coded of a current row SEkrepresented in FIG. 4A or 4B, such as for example the first row SE1.

In the course of a step C32, the unit UC or UCk tests whether thecurrent block is the first block (situated at the top and on the left)of the image IE which has been segmented into blocks in theaforementioned step C1.

If such is the case, in the course of a step C33, the entropy codingmodule MCE or MCEk undertakes an initialization of its state variables.According to the example represented, which uses the arithmetic codingdescribed previously, this entails an initialization of an intervalrepresentative of the probability of occurrence of a symbol contained inthe predetermined set of symbols. In a manner known per se, thisinterval is initialized with two bounds L and H, respectively lower andupper. The value of the lower bound L is fixed at 0, while the value ofthe upper bound is fixed at 1, thereby corresponding to the probabilityof occurrence of a first symbol from among all the symbols of thepredetermined set of symbols. The size R of this interval is thereforedefined at this juncture by R=H−L=1. The initialized interval isfurthermore partitioned conventionally into a plurality of predeterminedsub-intervals which are representative respectively of the probabilitiesof occurrence of the symbols of the predetermined set of symbols.

As a variant, if the entropy coding used is the LZW coding, atranslation table of strings of symbols is initialized, so that itcontains all the possible symbols once and only once.

If subsequent to the aforementioned step C32, the current block is notthe first block of the image IE, there is undertaken, in the course of astep C40 which will be described later in the subsequent description,the determination of the availability of the necessary previously codedand decoded blocks.

In the course of a step C34, there is undertaken the coding of the firstcurrent block MB1 of the first row SE1 represented in FIG. 4A or 4B.Such a step C34 comprises a plurality of sub-steps C341 to C348 whichwill be described hereinbelow.

In the course of a first sub-step C341 represented in FIG. 2B, there isundertaken the predictive coding of the current block MB1 by knowntechniques of intra and/or inter prediction, in the course of which theblock MB1 is predicted with respect to at least one previously coded anddecoded block.

It goes without saying that other modes of intra prediction such asproposed in the H.264 standard are possible.

The current block MB1 can also be subjected to a predictive coding ininter mode, in the course of which the current block is predicted withrespect to a block arising from a previously coded and decoded image.Other types of prediction are of course conceivable. Among the possiblepredictions for a current block, the optimal prediction is chosenaccording to a bitrate distortion criterion well known to the personskilled in the art.

Said aforementioned predictive coding step makes it possible toconstruct a predicted block MBp₁ which is an approximation of thecurrent block MB₁. The information relating to this predictive codingwill subsequently be written to the stream F transmitted to the decoderDO. Such information comprises in particular the type of prediction(inter or intra), and if appropriate, the mode of intra prediction, thetype of partitioning of a block or macroblock if the latter has beensubdivided, the reference image index and the displacement vector usedin the inter prediction mode. This information is compressed by thecoder CO.

In the course of a following sub-step C342, there is undertaken thesubtraction of the predicted block MBp₁ from the current block MB₁ toproduce a residual block MBr₁.

In the course of a following sub-step C343, there is undertaken thetransformation of the residual block MBr₁ according to a conventionaldirect transformation operation such as for example a discrete cosinetransformation of DCT type, to produce a transformed block MBt₁.

In the course of a following sub-step C344, there is undertaken thequantization of the transformed block MBt₁ according to a conventionalquantization operation, such as for example a scalar quantization. Ablock of quantized coefficients MB_(q) is then obtained.

In the course of a following sub-step C345, there is undertaken theentropy coding of the block of quantized coefficients MBq₁. In thepreferred embodiment, this entails a CABAC entropy coding. Such a stepconsists in:

-   -   a) reading the symbol or symbols of the predetermined set of        symbols which are associated with said current block,    -   b) associating digital information, such as bits, with the        symbol(s) read.

In the aforementioned variant according to which the coding used is anLZW coding, a digital item of information corresponding to the code ofthe symbol in the current translation table is associated with thesymbol to be coded, and an update of the translation table is performed,according to a procedure known per se.

In the course of a following sub-step C346, there is undertaken thedequantization of the block MBq₁ according to a conventionaldequantization operation, which is the operation inverse to thequantization performed in step C344. A block of de-quantizedcoefficients MBDq₁ is then obtained.

In the course of a following sub-step C347, there is undertaken theinverse transformation of the block of de-quantized coefficients MBD_(q)which is the operation inverse to the direct transformation performed instep C343 hereinabove.

A decoded residual block MBDr₁ is then obtained.

In the course of a following sub-step C348, there is undertaken theconstruction of the decoded block MBD₁ by adding to predicted block MBp₁the decoded residual block MBDr₁. It should be noted that the latterblock is the same as the decoded block obtained on completion of themethod for decoding the image IE, which will be described further on inthe description. The decoded block MBD₁ is thus rendered available to beused by the coding unit UCk or any other coding unit forming part of thepredetermined number R of coding units.

On completion of the aforementioned coding step C34, the entropy codingmodule MCE or MCEk such as represented in FIG. 3B contains all theprobabilities such as progressively re-updated in tandem with the codingof the first block. These probabilities correspond to the variouselements of possible syntaxes and to the various associated codingcontexts.

Subsequent to the aforementioned coding step C34, a test is performed,in the course of a step C35, to determine whether the current block isthe jth block of this same row, where j is a predetermined value knownto the coder CO which is at least equal to 1.

If such is the case, in the course of a step C36 represented in FIG. 2B,the set of probabilities calculated for the jth block is stored in thebuffer memory MT of the coder CO such as represented in FIG. 3A or 3Band in FIGS. 4A and 4B, the size of said memory being adapted forstoring the calculated number of probabilities.

In the course of a step C37 represented in FIG. 2B, the coding unit UCor UCk tests whether the current block of the row SEk which has justbeen coded is the last block of the image IE. Such a step is alsoimplemented if in the course of step C35, the current block is not thejth block of the row SE1.

If the current block is the last block of the image IE, in the course ofa step C38, the coding method is terminated.

If such is not the case, there is undertaken, in the course of step C39,the selection of the next block MB_(i) to be coded in accordance withthe order of traversal represented by the arrow PS in FIG. 4A or 4B.

In the course of a step C40 represented in FIG. 2B, there is undertakenthe determination of the availability of previously coded and decodedblocks which are necessary for coding the current block MB_(i).

If this is the first row SE1, such a step consists in verifying theavailability of at least one block situated on the left of the currentblock to be coded MB_(i). However, having regard to the order oftraversal PS chosen in the embodiment represented in FIG. 4A or 4B, theblocks are coded one after the other over a row SEk under consideration.Consequently, the left coded and decoded block is always available (withthe exception of the first block of a row). In the example representedin FIG. 4A or 4B, this is the block situated immediately to the left ofthe current block to be coded.

If this is a different row SEk from the first row, said determinationstep furthermore consists in verifying whether a predetermined number N′of blocks situated in the previous row SEk−1, for example the two blockssituated respectively above and above and to the right of the currentblock, are available for the coding of the current block, that is to sayif they have already been coded and then decoded by the coding unit UCor UCk−1.

As this test step is apt to slow down the coding method, in analternative manner in accordance with the invention, in the case wherethe coding of the rows is of parallel type, a clock CLK represented inFIG. 3C is adapted to synchronize the advance of the coding of theblocks so as to guarantee the availability of the two blocks situatedrespectively above and above and to the right of the current block,without it being necessary to verify the availability of these twoblocks. Thus, a coding unit UCk always begins to code the first blockwith a shift of a predetermined number N′ (with for example N′=2) ofcoded and decoded blocks of the previous row SEk−1 which are used forthe coding of the current block. From a software point of view, theimplementation of such a clock makes it possible to noticeablyaccelerate the time to process the blocks of the image IE in the coderCO.

In the course of a step C41 represented in FIG. 2B, a test is performedto determine whether the current block is the first block of the row SEkunder consideration.

If such is the case, in the course of a step C42, there is undertakenthe reading in the buffer memory MT solely of the symbol occurrenceprobabilities calculated during the coding of the jth block of theprevious row SEk−1.

According to a first variant represented in FIG. 4A, the jth block isthe first block of the previous row SEk−1 (j=1). Such a reading consistsin replacing the probabilities of the CABAC coder with those present inthe buffer memory MT. Entailing as it does the respective first blocksof the second, third and fourth rows SE2, SE3 and SE4, this reading stepis depicted in FIG. 4A by the arrows represented by slender lines.

According to a second variant of the aforementioned step C43 which isillustrated in FIG. 4B, the jth block is the second block of theprevious row SEk−1 (j=2). Such a reading consists in replacing theprobabilities of the CABAC coder with those present in the buffer memoryMT. Entailing as it does the respective first blocks of the second,third and fourth rows SE2, SE3 and SE4, this reading step is depicted inFIG. 4B by the arrows represented by slender dashed lines.

Subsequent to step C42, the current block is coded and then decoded byrepetition of steps C34 to C38 described above.

If subsequent to the aforementioned step C41, the current block is notthe first block of the row SEk under consideration, there isadvantageously not undertaken the reading of the probabilities arisingfrom the previously coded and decoded block which is situated in thesame row SEk, that is to say the coded and decoded block situatedimmediately to the left of the current block, in the examplerepresented. Indeed, having regard to the sequential traversal PS forreading the blocks situated in the same row, as represented in FIG. 4Aor 4B, the symbol occurrence probabilities present in the CABAC coder atthe moment of beginning the coding of the current block are exactlythose which are present after coding/decoding of the previous block inthis same row. Consequently, in the course of a step C43 represented inFIG. 2B, there is undertaken the learning of the probabilities of symboloccurrence for the entropy coding of said current block, thesecorresponding solely to those which have been calculated for saidprevious block in the same row, as represented by the double solidarrows in FIG. 4A or 4B.

Subsequent to step C43, the current block is coded and then decoded byrepetition of steps C34 to C38 described above.

A test is performed thereafter, in the course of a step C44, todetermine whether the current block is the last block of the row SEkunder consideration.

If this is not the case, subsequent to step C44, step C39 of selectingthe next block MB_(i) to be coded is implemented again.

If the current block is the last block of the row SEk underconsideration, in the course of a step C45, the coding device CO of FIG.3A or 3C performs an emptying as mentioned above in the description. Forthis purpose, the coding unit UCk transmits to the correspondingsub-stream generating module MGSFk the entirety of the bits which havebeen associated with the symbol(s) read during the coding of each blockof said row SEk under consideration, in such a way that the module MGSFkwrites, to the data sub-stream Fm containing a binary trainrepresentative of the coded blocks of said row SEk under consideration,said entirety of bits. Such an emptying is symbolized in FIGS. 4A and 4Bby a triangle at the end of each row SEk.

In the course of a step C46 represented in FIG. 2B, the coding unit UCor UCk performs a step identical to the aforementioned step C33, that isto say again initializes the interval representative of the probabilityof occurrence of a symbol contained in the predetermined set of symbols.Such a reinitialization is depicted in FIGS. 4A and 4B by a black dot atthe start of each row SEk.

The benefit of performing steps C45 and C46 at this level of the codingis that during the coding of the next block processed by the coding unitUC or a coding unit UCk, the coder CO is in an initialized state. Thus,as will be described further on in the description, it becomes possiblefor a decoding unit working in parallel to directly decode thecompressed stream F from this point, since it suffices for it to be inthe initialized state.

An Embodiment of the Decoding Part

An embodiment of the decoding method according to the invention will nowbe described, in which the decoding method is implemented in a softwareor hardware manner by modifications of a decoder initially compliantwith the H.264/MPEG-4 AVC standard.

The decoding method according to the invention is represented in theform of an algorithm comprising steps D1 to D4, represented in FIG. 5A.

According to the embodiment of the invention, the decoding methodaccording to the invention is implemented in a decoding device DOrepresented in FIG. 6A.

With reference to FIG. 5A, the first decoding step D1 is theidentification in said stream F of the L sub-streams F1, F2, . . . , Fm,. . . , FL containing respectively the P subsets SE1, SE2, . . . , SEk,. . . , SEP of previously coded blocks or macro-blocks MB_(i) asrepresented in FIG. 4A or 4B. For this purpose, each sub-stream Fm inthe stream F is associated with an indicator intended to allow thedecoder DO to determine the location of each sub-stream Fm in the streamF. As a variant, on completion of the aforementioned coding step C3, thecoder CO orders the sub-streams F1, F2, . . . , Fm, . . . , FL in thestream F, in the order expected by the decoder DO, thereby avoiding theinsertion into the stream F of the sub-stream indicators. Such aprovision thus makes it possible to reduce the cost in terms of bitrateof the data stream F.

In the example represented in FIG. 4A or 4B, said blocks MB have asquare shape and all have the same size. Depending on the size of theimage which is not necessarily a multiple of the size of the blocks, thelast blocks on the left and the last blocks at the bottom may not besquare. In an alternative embodiment, the blocks can be for example ofrectangular size and/or not aligned with one another.

Each block or macroblock can moreover itself be divided into sub-blockswhich are themselves subdividable.

Such an identification is performed by a stream extraction module EXDOsuch as represented in FIG. 6A.

In the example represented in FIG. 4A or 4B, the predetermined number Pis equal to 6 but only four subsets SE1, SE2, SE3, SE4 are representeddashed, for the sake of clarity of the figures.

With reference to FIG. 5A, the second decoding step D2 is the decodingof each of said subsets of blocks SE1, SE2, SE3 and SE4, the blocks of asubset under consideration being coded according to a predeterminedsequential order of traversal PS. In the example represented in FIG. 4Aor 4B, the blocks of a current subset SEk (1≤k≤P) are decoded one afteranother, from left to right, as indicated by the arrow PS. On completionof step D2, the decoded subsets of blocks SED1, SED2, SED3, . . . ,SEDk, . . . , SEDP are obtained.

Such a decoding can be of sequential type and, consequently, beperformed with the aid of a single decoding unit.

However, so as to be able to benefit from a multiplatform decodingarchitecture, the decoding of the subsets of blocks is of parallel typeand is implemented by a number R of decoding units UDk (1≤k≤R), with forexample R=4 as represented in FIG. 6A. Such a provision thus allows asubstantial acceleration of the decoding method. In a manner known perse, the decoder DO comprises a buffer memory MT which is adapted tocontain the symbol occurrence probabilities such as progressivelyreupdated in tandem with the decoding of a current block.

As represented in greater detail in FIG. 6B, each of the decoding unitsUDk comprises:

-   -   a module for entropy decoding of said current block by learning        at least one probability of symbol occurrence calculated for at        least one previously decoded block, denoted MDEk,    -   a module for predictive decoding of a current block with respect        to said previously decoded block, denoted MDPk.

The predictive decoding module SUDPk is able to perform a predictivedecoding of the current block, according to conventional predictiontechniques, such as for example in Intra and/or Inter mode.

The entropy decoding module MDEk is for its part of CABAC type, butmodified according to the present invention as will be described furtheron in the description.

As a variant, the entropy decoding module MDEk could be a Huffmandecoder known per se.

In the example represented in FIG. 4A or 4B, the first unit UD1 decodesthe blocks of the first row SE1, from left to right. When it reaches thelast block of the first row SE1, it passes to the first block of the(n+1)^(th) row, here the 5^(th) row, etc. The second unit UC2 decodesthe blocks of the second row SE2, from left to right. When it reachesthe last block of the second row SE2, it passes to the first block ofthe (n+2)^(th) row, here the 6^(th) row, etc. This traversal is repeatedas far as the unit UD4, which decodes the blocks of the fourth row SE4,from left to right. When it reaches the last block of the first row, itpasses to the first block of the (n+4)^(th) row, here the 8^(th) row,and so on and so forth until the last block of the last identifiedsub-stream is decoded.

Other types of traversal than that which has just been describedhereinabove are of course possible. For example, each decoding unitcould process not nested rows, as explained hereinabove, but nestedcolumns. It is also possible to traverse the rows or columns in eitherdirection.

With reference to FIG. 5A, the third decoding step D3 is thereconstruction of a decoded image ID on the basis of each decoded subsetSED1, SED2, . . . , SEDk, . . . , SEDP obtained in the decoding step D2.More precisely, the decoded blocks of each decoded subset SED1, SED2, .. . , SEDk, . . . , SEDP are transmitted to an image reconstruction unitURI such as represented in FIG. 6A. In the course of this step D3, theunit URI writes the decoded blocks into a decoded image as these blocksbecome available.

In the course of a fourth decoding step D4 represented in FIG. 5A, afully decoded image ID is delivered by the unit URI represented in FIG.6A.

The various specific sub-steps of the invention, such as are implementedduring the aforementioned step D2 of parallel decoding, in a decodingunit UDk, will now be described with reference to FIG. 5B.

In the course of a step D21, the decoding unit UDk selects as currentblock the first block to be decoded of the current row SEk representedin FIG. 4A or 4B.

In the course of a step D22, the decoding unit UDk tests whether thecurrent block is the first block of the decoded image, in this instancethe first block of the sub-stream F1.

If such is the case, in the course of a step D23, the entropy decodingmodule MDE or MDEk undertakes an initialization of its state variables.According to the example represented, this entails an initialization ofan interval representative of the probability of occurrence of a symbolcontained in the predetermined set of symbols.

As a variant, if the entropy decoding used is LZW decoding, atranslation table of strings of symbols is initialized, so that itcontains all the possible symbols once and only once. Step D23 beingidentical to the aforementioned coding step C33, it will not bedescribed subsequently.

If subsequent to the aforementioned step D22, the current block is notthe first block of the decoded image ID, there is undertaken, in thecourse of a step D30 which will be described later in the subsequentdescription, the determination of the availability of the necessarypreviously decoded blocks.

In the course of a step D24, there is undertaken the decoding of thefirst current block MB1 of the first row SE1 represented in FIG. 4A or4B. Such a step D24 comprises a plurality of sub-steps D241 to D246which will be described hereinbelow.

In the course of a first sub-step D241, there is undertaken the entropydecoding of the syntax elements related to the current block. Such astep consists mainly in:

-   -   a) reading the bits contained in the sub-stream associated with        said first row SE1,    -   b) reconstructing the symbols on the basis of the bits read.

In the aforementioned variant according to which the decoding used is anLZW decoding, a digital item of information corresponding to the code ofthe symbol in the current translation table is read, the symbol isreconstructed on the basis of the code read and an update of thetranslation table is performed, according to a procedure known per se.

More precisely, the syntax elements related to the current block aredecoded by the CABAC entropy decoding module MDE1 such as represented inFIG. 6B. The latter decodes the sub-stream of bits F1 of the compressedfile to produce the syntax elements, and, at the same time, reupdatesits probabilities in such a way that, at the moment at which the latterdecodes a symbol, the probabilities of occurrence of this symbol areidentical to those obtained during the coding of this same symbol duringthe aforementioned entropy coding step C345.

In the course of a following sub-step D242, there is undertaken thepredictive decoding of the current block MB1 by known techniques ofintra and/or inter prediction, in the course of which the block MB1 ispredicted with respect to at least one previously decoded block.

It goes without saying that other modes of intra prediction such asproposed in the H.264 standard are possible.

In the course of this step, the predictive decoding is performed withthe aid of the syntax elements decoded in the previous step andcomprising in particular the type of prediction (inter or intra), and ifappropriate, the mode of intra prediction, the type of partitioning of ablock or macroblock if the latter has been subdivided, the referenceimage index and the displacement vector used in the inter predictionmode.

Said aforementioned predictive decoding step makes it possible toconstruct a predicted block MBp₁.

In the course of a following sub-step D243, there is undertaken theconstruction of a quantized residual block MBq₁ with the aid of thepreviously decoded syntax elements.

In the course of a following sub-step D244, there is undertaken thedequantization of the quantized residual block MBq₁ according to aconventional dequantization operation which is the operation inverse tothe quantization performed in the aforementioned step C344, to produce adecoded dequantized block MBDt₁.

In the course of a following sub-step D245, there is undertaken theinverse transformation of the dequantized block MBDt₁ which is theoperation inverse to the direct transformation performed in step C343hereinabove. A decoded residual block MBDr₁ is then obtained.

In the course of a following sub-step D246, there is undertaken theconstruction of the decoded block MBD₁ by adding to predicted block MBp₁the decoded residual block MBDr₁. The decoded block MBD₁ is thusrendered available to be used by the decoding unit UD1 or any otherdecoding unit forming part of the predetermined number N of decodingunits.

On completion of the aforementioned decoding step D246, the entropydecoding module MDE1 such as represented in FIG. 6B contains all theprobabilities such as progressively reupdated in tandem with thedecoding of the first block. These probabilities correspond to thevarious elements of possible syntaxes and to the various associateddecoding contexts.

Subsequent to the aforementioned decoding step D24, a test is performed,in the course of a step D25, to determine whether the current block isthe jth block of this same row, where j is a predetermined value knownto the decoder DO which is at least equal to 1.

If such is the case, in the course of a step D26, the set ofprobabilities calculated for the jth block is stored in the buffermemory MT of the decoder DO such as represented in FIG. 6A and in FIG.4A or 4B, the size of said memory being adapted for storing thecalculated number of probabilities.

In the course of a step D27, the unit UDk tests whether the currentblock which has just been decoded is the last block of the lastsub-stream.

If such is the case, in the course of a step D28, the decoding method isterminated.

If such is not the case, there is undertaken, in the course of step D29,the selection of the next block MB_(i) to be decoded in accordance withthe order of traversal represented by the arrow PS in FIG. 4A or 4B.

If in the course of the aforementioned step D25, the current block isnot the jth block of the row SEDk under consideration, step D27hereinabove is undertaken.

In the course of a step D30 which follows the aforementioned step D29,there is undertaken the determination of the availability of previouslydecoded blocks which are necessary for decoding the current blockMB_(i). Having regard to the fact that this entails a parallel decodingof the blocks by different decoding units UDk, it may happen that theseblocks have not been decoded by the decoding unit assigned to thedecoding of these blocks and that they are therefore not yet available.Said determination step consists in verifying whether a predeterminednumber N′ of blocks situated in the previous row SEk−1, for example thetwo blocks situated respectively above and above and to the right of thecurrent block, are available for the decoding of the current block, thatis to say whether they have already been decoded by the decoding unitUDk−1 assigned to their decoding. Said determination step also consistsin verifying the availability of at least one block situated on the leftof the current block to be decoded MB_(i). However, having regard to theorder of traversal PS chosen in the embodiment represented in FIG. 4A or4B, the blocks are decoded one after the other over a row SEk underconsideration. Consequently, the left decoded block is always available(with the exception of the first block of a row). In the examplerepresented in FIG. 4A or 4B, this entails the block situatedimmediately to the left of the current block to be decoded. For thispurpose, only the availability of the two blocks situated respectivelyabove and above and to the right of the current block is tested.

As this test step is apt to slow down the decoding method, in analternative manner in accordance with the invention, a clock CLKrepresented in FIG. 6A is adapted to synchronize the advance of thedecoding of the blocks so as to guarantee the availability of the twoblocks situated respectively above and above and to the right of thecurrent block, without it being necessary to verify the availability ofthese two blocks. Thus, as represented in FIG. 4A or 4B, a decoding unitUDk always begins to decode the first block with a shift of apredetermined number N′ (here N′=2) of decoded blocks of the previousrow SEk−1 which are used for the decoding of the current block. From asoftware point of view, the implementation of such a clock makes itpossible to noticeably accelerate the time to process the blocks of eachsubset SEk in the decoder DO.

In the course of a step D31, a test is performed to determine whetherthe current block is the first block of the row SEk under consideration.

If such is the case, in the course of a step D32, there is undertakenthe reading in the buffer memory MT solely of the symbol occurrenceprobabilities calculated during the decoding of the jth block of theprevious row SEk−1.

According to a first variant represented in FIG. 4A, the jth block isthe first block of the previous row SEk−1 (j=1). Such a reading consistsin replacing the probabilities of the CABAC decoder with those presentin the buffer memory MT. Entailing as it does the respective firstblocks of the second, third and fourth rows SE2, SE3 and SE4, thisreading step is depicted in FIG. 4A by the arrows represented by slenderlines.

According to a second variant of the aforementioned step D32 which isillustrated in FIG. 4B, the jth block is the second block of theprevious row SEk−1 (j=2). Such a reading consists in replacing theprobabilities of the CABAC decoder with those present in the buffermemory MT. Entailing as it does the respective first blocks of thesecond, third and fourth rows SE2, SE3 and SE4, this reading step isdepicted in FIG. 4B by the arrows represented by slender dashed lines.

Subsequent to step D32, the current block is decoded by repetition ofsteps D24 to D28 described above.

If subsequent to the aforementioned step D31, the current block is notthe first block of the row SEk under consideration, there isadvantageously not undertaken the reading of the probabilities arisingfrom the previously decoded block which is situated in the same row SEk,that is to say the decoded block situated immediately to the left of thecurrent block, in the example represented. Indeed, having regard to thesequential traversal PS for reading the blocks situated in the same row,as represented in FIG. 4A or 4B, the symbol occurrence probabilitiespresent in the CABAC decoder at the moment of beginning the decoding ofthe current block are exactly that which are present after decoding ofthe previous block in this same row.

Consequently, in the course of a step D33, there is undertaken thelearning of the probabilities of symbol occurrence for the entropydecoding of said current block, said probabilities corresponding solelyto those which have been calculated for said previous block in the samerow, as represented by the double solid arrows in FIG. 4A or 4B.

Subsequent to step D33, the current block is decoded by repetition ofsteps D24 to D28 described above.

A test is performed thereafter, in the course of a step D34, todetermine whether the current block is the last block of the row SEkunder consideration.

If such is not the case, subsequent to step D34, step D29 of selectingthe next block MB_(i) to be coded is implemented again.

If the current block is the last block of the row SEk underconsideration, in the course of a step D35, the decoding unit UDkperforms a step identical to the aforementioned step D23, that is to sayagain initializes the interval representative of the probability ofoccurrence of a symbol contained in the predetermined set of symbols.Such a reinitialization is depicted in FIGS. 4A and 4B by a black dot atthe start of each row SEk.

Thus, the decoder DO is in an initialized state at each start of row,thereby allowing great flexibility from the point of view of choosingthe level of parallelism of decoding and optimization of the processingtime on decoding.

In the exemplary coding/decoding diagram represented in FIG. 7A, thecoder CO comprises a single coding unit UC, as represented in FIG. 3A,while the decoder DO comprises six decoding units.

The coding unit UC codes the rows SE1, SE2, SE3, SE4, SE5 and SE6sequentially. In the example represented, rows SE1 to SE4 are fullycoded, row SE5 is in the course of being coded and row SE6 has not yetbeen coded. Having regard to the sequentiality of the coding, the codingunit UC is adapted for delivering a stream F which contains thesub-streams F1, F2, F3, F4 ordered one following after another, in theorder of coding of the rows SE1, SE2, SE3 and SE4. For this purpose, thesub-streams F1, F2, F3 and F4 are symbolized with the same hatching asthat which respectively symbolizes the coded rows SE1, SE2, SE3, SE4. Byvirtue of the emptying steps at the end of the coding of said coded rowsand of the reinitialization of the interval of probabilities oncommencing the coding or the decoding of the next row to becoded/decoded, the decoder DO, each time that it reads a sub-stream soas to decode it, is in an initialized state and can therefore, in anoptimal manner, decode in parallel the four sub-streams F1, F2, F3, F4with decoding units UD1, UD2, UD3 and UD4 which can for example beinstalled on four differentplatforms.

In the exemplary coding/decoding diagram represented in FIG. 7B, thecoder CO comprises two coding units UC1 and UC2, as represented in FIG.3C, while the decoder DO comprises six decoding units.

The coding unit UC1 sequentially codes the rows of odd rank SE1, SE3 andSE5, while the coding unit UC2 sequentially codes the rows of even rankSE2, SE4 and SE6. For this purpose, rows SE1, SE3 and SE5 exhibit awhite background, while rows SE2, SE4 and SE6 exhibit a dottedbackground. In the example represented, rows SE1 to SE4 are fully coded,row SE5 is in the course of being coded and row SE6 has not yet beencoded. Having regard to the fact that the coding performed is ofparallel type of level 2, the coding unit UC1 is adapted for deliveringa sub-stream F_(2n+1) decomposed into two parts F1 and F3 obtainedsubsequent to the coding respectively of rows SE1 and SE3, while thecoding unit UC2 is adapted for delivering a sub-stream F_(2n) decomposedinto two parts F2 and F4 obtained subsequent to the coding respectivelyof rows SE2 and SE4. The coder CO is therefore adapted for transmittingto the decoder DO a stream F which contains the juxtaposition of the twosub-streams F_(2n+1) and F_(2n) and therefore an ordering of thesub-streams F1, F3, F2, F4 which differs from that represented in FIG.7A. For this purpose, the sub-streams F1, F2, F3 and F4 are symbolizedwith the same hatching as that which respectively symbolizes the codedrows SE1, SE2, SE3, SE4, the sub-streams F1 and F3 exhibiting a whitebackground (coding of the rows of odd rank) and the sub-streams F2 andF4 exhibiting a dotted background (coding of the rows of even rank).

With respect to the advantages mentioned in conjunction with FIG. 7A,such a coding/decoding diagram furthermore presents the advantage ofbeing able to employ a decoder whose level of parallelism of decoding iscompletely independent of the level of parallelism of the coding,thereby making it possible to optimize the operation of a coder/decoderstill further.

1. A method for coding at least one image comprising the steps of:segmenting (C1) the image into a plurality of blocks (MB) able tocontain symbols belonging to a predetermined set of symbols, grouping(C2) of blocks into a predetermined number (P) of subsets of blocks(SE1, SE2, . . . , SEk, . . . , SEP), coding (C3), by means of anentropy coding module, of each of said subsets of blocks, by associatingdigital information with the symbols of each block of a subset underconsideration, said coding step comprising, for the first block of theimage, a sub-step (C33) of initializing state variables of said entropycoding module, generation (C4) of at least one data sub-stream (F1)representative of at least one (SE1) of said coded subsets of blocks,said coding method being characterized in that: in the case where thecurrent block is the first block to be coded of a subset underconsideration, determination (C42) of probabilities of symbol occurrencefor said first current block, said probabilities being those which havebeen determined for a coded and decoded predetermined block of at leastone other subset, in the case where the current block is the last codedblock of the subset under consideration: writing (C45), in thesub-stream representative of said at least subset under consideration,of the entirety of the digital information which has been associatedwith the symbols during the coding of the blocks of said subset underconsideration, Implementation (C46) of said initialization sub-step.